1. Field of the Invention:
This invention relates to a self-drilling screw having an integral drill bit formed at a lower end of a threaded shank to drill an entry hole in a workpiece, and more particularly to improvements in the self-drilling screw of the type that has the drill bit shaped through cold forging such as pinch pointing.
2. Description of the Prior Art:
Recently, a drill bit of a self-drilling screw is, in many instances, formed not by machining but by cold forging (typically by pinch pointing) because the latter is more advantageous in mass production.
In general, the self-drilling screws with a forged drill bit can be roughly classified into two types. In the first type, the terminal end of the drill bit is forged into a pin point shape, as disclosed for example in U.S. Pat. No. 3,710,676 to Ringland. In the second type, that of the drill bit is forged into a chisel edge shape, as disclosed for example in U.S. Pat. Nos. 3,395,603 to E. J. Skierski and 4,407,620 to Shinjo. Each of the two types has its particular advantages and disadvantages.
The pin pointed drill bit is generally advantageous in that it easily permits an attempt to drill an entry hole correctly at an intended spot of application on a blank workpiece because its pin pointed drill end is in point-contact with the workpiece surface at the initial phase of screwing operation, whereby the drill end is easily embedded in the workpiece so as to prevent an undesirable lateral moving of the screw from the intended spot of application on the workpiece, hereinafter referred to as "wandering" or "wandering phenomenon". However, this type drill bit is generally disadvantageous in that its pin pointed drill end damages easily to lose its expected function, and, as is well known in the art, an initial intrusion of the drill end into a workpiece is slower in speed than that of the chisel edged drill bit.
In contrast thereto, the chisel edged drill bit is generally advantageous in that its drill end is less damageable than the pin pointed drill end and, in addition, the initial intrusion of the drill end into a workpiece is more rapidly carried out than that of the pin pointed drill bit. However, this type is generally disadvantageous in that the chisel edged configuration causes more or less the undesirable wandering of the screw on the workpiece surface when the screw is forced thereagainst and driven into rotation by a power driver, because its drill end is not pin pointed for easy penetration but chisel edged so as to provide a line-contact with the workpiece surface at the initial phase of the operation.
For ease of further discussion of particular disadvantages resulting from the prior art configuration of the chisel edged drill bit, description is now made with reference to FIGS. 15 to 18 of the accompanying drawings, in which the drill bit 1 is illustrated as having a pair of inclined flat intrusion faces 3 disposed symmetrically with respect to an axis 2 of the screw, that is an axis of rotation of the screw. The flat intrusion faces 3 terminate to share a common lower extremity E as shown in FIG. 18. In other words, the flat intrusion faces 3 meet each other along a line 7 (FIG. 16) of intersection at the lower extremity E to form a rectilinear chisel edge 7 extending transversely of the screw axis 2.
The drill bit 1 includes a pair of axial flutes 4 located symmetrically with respect to the screw axis 2. Each flute 4 is disposed adjacent to each of the intrusion faces 3 and extends upwardly from the lower extremity E. Each flute 4 has its one surface 4a extending to meet one of the intrusion faces 3 along a line 6 of intersection to form an oblique cutting edge 6 which is inclined with respect to the axis 2. A pair of the cutting edges 6 thus formed are oppositely disposed with respect to a vertical imaginary reference plane 5 which contains the axis 2, as shown in FIG. 16. One of the cutting edges 6 terminates at one end 6a of the chisel edge 7 while the other thereof at the other end 6b of the chisel edge 7. Thus, a length of the chisel edge 7 is defined by a distance L' between the opposite terminal points 6a, 6b of the pair of cutting edges 6, as seen in FIG. 15. One of the flutes 4 starts at the aforesaid terminal point 6a while the other thereof at the other terminal point 6b, as best shown in FIG. 15.
Since the chisel edge 7 is formed at the lower extremity E along the line of intersection of the two flat intrusion faces 3, a longitudinal section of a lower end portion of the drill bit 1 taken along any imaginary plane (for example a plane containing the line 18--18 in FIG. 15) that is transverse to the chisel edge line 7, is always of such a triangular configuration whose one side 3a is located on one side of the axis 2 while the other side 3b thereof on the opposite side of the axis 2. Naturally, an angle .alpha. defined by the side 3b and a workpiece surface Ws is less than 90.degree., as shown in FIG. 18. This means that the side 3b does not serve as a rake face but prevents introduction of produced chips into the flute 4, resulting in that the drilling capability is inferior.
Further, the prior art drill bit configuration does not eliminate the above discussed general disadvantage that the screw is easy to wander on a surface Ws of a workpiece W because the chisel edge 7 is in line-contact with the workpiece surface Ws at the initial phase of screwing operation.